The image displaying portion used in an image display includes a gas discharging display panel, for example a plasma display panel (hereunder referred to simply as "PDP"). The PDP excites the molecules of the gases sealed up inside by discharging between the electrodes (more specifically, it mixes up the xenon gas and neon gas, and brings assistance to the excitation of the xenon gas molecules), excites further the fluorescent substance coated inside by the ultraviolet radiation generated, thus emitting the visible light to display an image. The discharge and the like under these conditions however generate an electromagnetic wave which leaks out, though small in quantity.
To prevent this electromagnetic wave from leaking out, an optical filter provided on the frontal face of the PDP to intercept wavelengths in the near infrared region is endowed with an electromagnetic wave leakage attenuation feature. This function of preventing the electromagnetic wave from leaking out is achieved by providing, as shown in FIGS. 1 and 2, a filter base 11 consisting of a synthetic resin plate such as an acrylic, the base material of the optical filter (referred to simply as "filter"), with a web-like formed conductor (referred to simply as "conductive mesh 12"). As shown in FIG. 3, the conductor width and intervals of the grids of the conductive mesh 12 have been set to their optimal values so that the conductive mesh as grounded may cover the frequency range of the electromagnetic wave to be prevented from leaking out and not interfere with the light image. That is, the electric charge generated at the conductive mesh 12 by the electromagnetic wave is grounded by connecting the conductive mesh 12 to the enclosure of the PDP with the orientation of the mesh being set obliquely, as shown in FIGS. 2 and 3, so that the matrices of the PDP pixels are not covered by the mesh conductor and intercept the image light.
Note here that the PDP applies about 350 V of pulse voltage between the electrodes at a predetermined period required for write/erase (the data of all the pixels is erased all at once to write a new image data into respective pixels). Since the filter 2 is arranged in the vicinity of the frontal face of the PDP, the frontal glass of the PDP and the filter 2 come into a status of capacitive coupling, said pulse voltage for write/erase generates an electric charge during said required period at the conductive mesh 12 of the filter 2 through the capacitive coupling. Though this electric charge produces an instantaneous voltage (approx. 140 V max. as measured) between the conductive mesh 12 and ground due to the impedance of the grounded circuit, the voltage of the conductive mesh 12 as continued to the ground becomes 0 V. Because, on the other hand, the electric charge imparts itself to the portion 41 as surrounded by the grids of the conductive mesh 12 (where there exists an acrylic adhesive) as shown in FIG. 3, together with the generation of the instantaneous voltage (about 140 V), the charge remains even after the voltage of the conductive mesh 12 becomes 0 (zero). The charged portion being within point-blank range of the conductive mesh 12, surpasses the withstand voltage to discharge (sparks) instantaneously toward the conductive mesh 12 at the same time when the voltage of the mesh lowers down to 0 V.
When, for example, the PDP is an AC (alternating current) driven type, the image signal is of NTSC system, and one field is divided into six subfields for driving, then the repeated (required) frequency of the voltage generation of the conductive mesh 12 is about 360 Hz (60 fields.times.6 subfields=360 Hz) with the discharging repeated at about 360 Hz too, and accordingly the spark noise is heard as an abnormal noise.
The present invention, arrived at in light of the foregoing problematical points, is intended to keep any abnormal noise due to the discharging of the imparted charge from occurring by reducing the electric charge as generated at the conductive mesh 12 or else by preventing the charge from imparting itself between the grids of the conductive mesh 12 (region where the filter base exists, including the portion 41 surrounded by the grids themselves; the same will prevail for the following texts) when the pulse voltage as applied to the image displaying portion (the pulse voltage applied to the PDP for write/erase, for instance) generates an electric charge at the conductive mesh 12.